1. Field of the Invention
This invention relates to the recovery of substantially pure triethylenediamine; and, more particularly to a process for recovering a triethylenediamine solute in liquid propylene glycol solutions directly from crude triethylenediamine reaction product mixtures.
2. Prior Art
Triethylenediamine (TEDA) is a valuable commercial product, particularly as an accelerator or catalyst in conventional urethane systems employing a wide variety of isocyanates and polyols as reactants. Several methods for preparing triethylenediamine are well known. For example, one process is described by O. Hromatka et al. in Berichter, Vol. 76, pages 712-717 (1943), wherein triethylenediamine is obtained by the process of heating the dihydrochloride of N-(2-hydroxyethyl)piperazine. Another process involves the gaseous phase cyclization of N-hydroxyethyl piperazine vapor in the presence of a solid acid catalyst. Another well-known process is described in U.S. Pat. No. 3,080,371 to Spielberger et al which includes the liquid phase process of heating N-(2-hydroxyethyl)piperazine in the presence of a mono- or dicarboxylic acid catalyst at a temperature of about 230.degree.-350.degree. C.
Generally, such known processes result in the formation of crude reaction product mixtures containing the triethylenediamine, water, by-products such as piperazine and high molecular weight polymers, catalysts and solvents, if any are employed. Triethylenediamine is normally separated from the crude reaction product by fractional distillation followed by one or more crystallization steps. The substantially pure solid triethylenediamine thus recovered is then dissolved in a suitable solvent for use as a urethane catalyst.
These generally described conventional techniques for recovering triethylenediamine have several disadvantages. Pure triethylenediamine has a freezing point of 159.8.degree. C and a boiling point of 174.degree. C. Pure triethylenediamine thus is normally a liquid over a very narrow temperature range of 14.2.degree. C. In view of this fact it is extremely difficult to separate triethylenediamine from its crude reaction product mixtures by conventional techniques other than crystallization. For example, pure triethylenediamine cannot be readily separated from reaction mixtures by conventional distillation techniques. Further, triethylenediamine readily freezes in the distillation equipment including condensation apparatus, vent lines, and the like, causing equipment blockage problems. Solid, e.g., crystallized, triethylenediamine is also difficult to work with. For example, the crystalline material tends to hydrate. Further, the solid compound is toxic requiring the use of special handling equipment to minimize human exposure.
In as much as conventional urethane systems normally utilize liquid reaction components and the solid triethylenediamine is difficult to handle, store, and ship, the solid is normally dissolved in a suitable solvent, e.g., dipropylene glycol which is compatible with urethane systems. These triethylenediamine solutions have heretofore been prepared by initially obtaining a substantially pure solid triethylenediamine by the methods previously described herein and then dissolving the solid in a suitable solvent.
In the present invention, substantially pure triethylenediamine is recovered directly from the crude reaction product mixture without the need for crystallization. The substantially pure triethylenediamine is recovered in a liquid solution which is substantially free from reaction by-products and can be used directly as a catalyst in urethane systems. Triethylenediamine liquid solutions thus obtained negate the necessity of handling solid triethylenediamine.